Browsing by Subject "Dissociation"

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    Reductive Elimination Reactions in Gold(III) Complexes Leading to C(sp3)–X (X = C, N, P, O, Halogen) Bond Formation: Inner-Sphere vs SN2 Pathways
    (American Chemical Society, 2023-01-19) Portugués Rodríguez, Alejandro; Martínez-Nortes, Miguel Ángel; Bautista, Delia; González Herrero, Pablo; Gil Rubio, Juan; Química Inorgánica
    The reactions leading to the formation of C–heteroatom bonds in the coordination sphere of Au(III) complexes are uncommon, and their mechanisms are not well known. This work reports on the synthesis and reductive elimination reactions of a series of Au(III) methyl complexes containing different Au–heteroatom bonds. Complexes [Au(CF3)(Me)(X)(PR3)] (R = Ph, X = OTf, OClO3, ONO2, OC(O)CF3, F, Cl, Br; R = Cy, X = Me, OTf, Br) were obtained by the reaction of trans-[Au(CF3)(Me)2(PR3)] (R = Ph, Cy) with HX. The cationic complex cis-[Au(CF3)(Me)(PPh3)2]OTf was obtained by the reaction of [Au(CF3)(Me)(OTf)(PPh3)] with PPh3. Heating these complexes led to the reductive elimination of MeX (X = Me, Ph3P+, OTf, OClO3, ONO2, OC(O)CF3, F, Cl, Br). Mechanistic studies indicate that these reductive elimination reactions occur either through (a) the formation of tricoordinate intermediates by phosphine dissociation, followed by reductive elimination of MeX, or (b) the attack of weakly coordinating anionic (TfO– or ClO4–) or neutral nucleophiles (PPh3 or NEt3) to the Au-bound methyl carbon. The obtained results show for the first time that the nucleophilic substitution should be considered as a likely reductive elimination pathway in Au(III) alkyl complexes.
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    Unfolding and refolding in vitro of a tetrameric, a-helical membrane protein: the prokaryotic potassium channel KcsA
    (American Chemical Society, 2005-10-06) Barrera Olivares, Francisco Nicolás; Renart Pérez, María Lourdes; Molina Gallego, María Luisa; Poveda Larrosa, José Antonio; Encinar Hidalgo, José Antonio; Fernández Carvajal, Asia María; Neira Faleiro, José Luis; González Ros, José Manuel; Bioquímica y Biología Molecular B e Inmunología
    2,2,2-Trifluoroethanol (TFE) effectively destabilizes the otherwise highly stable tetrameric structure of the potassium channel KcsA, a predominantly α-helical membrane protein [Valiyaveetil, F. I., Zhou, Y., and MacKinnon, R. (2002) Biochemistry 41, 10771−10777]. Here, we report that the effects on the protein structure of increasing concentrations of TFE in detergent solution include two successive protein concentration-dependent, cooperative transitions. In the first of such transitions, occurring at lower TFE concentrations, the tetrameric KcsA simultaneously increases the exposure of tryptophan residues to the solvent, partly loses its secondary structure, and dissociates into its constituent subunits. Under these conditions, simple dilution of the TFE permits a highly efficient refolding and tetramerization of the protein in the detergent solution. Moreover, following reconstitution into asolectin giant liposomes, the refolded protein exhibits nativelike potassium channel activity, as assessed by patch-clamp methods. Conversely, the second cooperative transition occurring at higher TFE concentrations results in the irreversible denaturation of the protein. These results are interpreted in terms of a protein and TFE concentration-dependent reversible equilibrium between the folded tetrameric protein and partly unfolded monomeric subunits, in which folding and oligomerization (or unfolding and dissociation in the other direction of the equilibrium process) are seemingly coupled processes. At higher TFE concentrations this is followed by the irreversible conversion of the unfolded monomers into a denatured protein form.